Acid transaction theory

What is? ACID

A transaction has four basic characteristics , That's what we often say （ACID）：

1. Atomicity（ Atomicity ）： A business is an indivisible whole , All operations within the transaction are either successful , Or it's a total failure .
2. Consistency（ Uniformity ）： Before and after implementation , Data from one state to another must be consistent （A towards B Transfer accounts , Cannot appear A Buckle the money ,B Has received ）.
3. Isolation（ Isolation, ）： Multiple concurrent transactions are isolated from each other , Don't interfere with each other .
4. Durability（ persistence ）： After the transaction completes , Changes to the database are permanent , Cannot be rolled back .

ACID By what guarantee ？

With MySQL For example ：

A Atomicity from undo log Log guarantees , It records the log information that needs to be rolled back , When the transaction is rolled back, undo the successfully executed sql

C Uniformity It is generally guaranteed by the code level

I Isolation, from MVCC To guarantee

D persistence By memory +redo log To guarantee ,mysql Modify data both in memory and redo log Record this operation , The transaction is committed through redo log Brush set , When it goes down, you can start from redo log recovery

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